Hurray For Plutonium!

Wait a minute! Isn’t plutonium that nasty stuff they make bombs from? Radioactive? The stuff Doc Brown stole from Libyans to fuel the Back to the Future DeLorean time machine? Well, yes…but there’s more to it than that.

Plutonium exists as several different isotopes, forms of a single element that differ only in the number of neutrons in their atomic nuclei. Their mass number is the total number of protons and neutrons. The isotope of plutonium that is used in nuclear weapons is plutonium-239, but the isotope we want to talk about is plutonium-238 (Pu-238).

Pu-238 is used as a power source—not for time machines, but for spacecraft. Two of its characteristics make it especially suitable for that purpose.

Almost all of the radiation emitted by Pu-238 is of a type that is easily stopped by minimal shielding. That translates to less weight needed to shield the rest of the spacecraft from this radiation.

It has an almost ideal half-life of 88 years. This is both long enough and short enough: long enough to last for decades on a long space mission, and short enough to provide lots of energy from a relatively small amount of material. Think for a minute about how the half-life of a radioactive material affects its activity. If half of it decays (emitting energy as it does so) in 100 years, that is a lot more energy than if the same amount of material takes a million years to decay. Shorter half-lives translate into higher activities.

The way the plutonium is used in a spacecraft is nothing so complicated as a nuclear reactor—a much, much simpler mechanism is used. There is so much activity coming from the plutonium that it is actually hot: not radioactively “hot”, but hot as in having a high temperature. In fact, it will glow from its own self-generated heat, as seen in this image of a pellet of plutonium oxide.

That heat can be turned into electrical current by the use of thermocouples, simple electrical devices that create a current flow when their two ends are at different temperatures. These, along with the plutonium oxide pellets, are incorporated into Radioisotope Thermoelectric Generators (RTGs) that have been the power source for numerous space missions.

The fins allow the heat to dissipate to the vacuum of space, the “heat sink” necessary to create the maximum temperature difference in the thermocouples and therefore the maximum current flow. GPHS in this image stands for general purpose heat source. Perhaps NASA just doesn’t want to print the word “plutonium”!

Any long-term mission to the outer solar system must use RTGs. Fuel cells or batteries can’t last that long, and they are far more complex devices. Solar power for solar cells drops off rapidly; at Mars the solar flux is already only 43% that of Earth. The New Horizons spacecraft on its way to a 2015 encounter with Pluto is shown below, with its RTG units prominently jutting from the craft on the left.

Pu-238 can be separated from spent fuel rods coming from commercial nuclear power plants, but that would require a difficult separation of this particular isotope from others of plutonium. It is prepared instead by separating another element found in irradiated fuel rods, neptunium-237. For those nuclear chemists among you, the process is Np-237 + n → Np-238 (β decay 2.1 d) → Pu-238.

The exploration of space has received several gifts as side effects of the Cold War, from rockets necessary to launch into space to miniaturized electronics that can survive harsh conditions. Among these side effects was the production of plutonium that could be used in RTGs, but such production was halted in 1988 with the end of the Cold War. As our supply dwindled, we purchased Pu-238 from the Russians, and the RTG currently powering the Curiosity rover on Mars is fueled with Russian plutonium. The Russians stopped producing it in 2010, however.

As in seemingly anything involving Congressional approval, politics got in the way of restarting American production, but in March of this year NASA’s planetary science division head Jim Green announced that production of Pu-238 by the U.S. Department of Energy is currently in the test phases leading up to a restart of full scale production. NASA should have enough to continue its exploration of the solar system.

So, if you give a thought to plutonium at all, keep in mind that it is ultimately responsible for images such as these of Mars from Curiosity, and Saturn and its moon Titan from Cassini.